Is it a Subaru? Mine is currently doing that when I go through a long turn in a highway interchange. I'm sure I need to add fluid but it's on the edge where it only shows up when the force of gravity is pulling the remaining fluid a certain way.

Maybe it's a combination of the "bend" and the "long slow" part.

If you are making a turn the centrifugal force will push the fluid away from the axis of rotation. There is likely a level sensor only on one side of the tank, so the turn might push the fluid away from the sensor enough to trigger a warning.

The sensor likely has a time component to avoid triggering every time you make a turn, but if this bend is long enough, maybe the fluid is displaced long enough that it overcomes that minimum time.

Perhaps it's long and slow enough to make the fluid move to a certain point and stay there long enough for the light/alarm to come on. If it's electronic vs solely mechanical, I'm sure they've got some smarts so that certain changes are ignored for a period of time (incline/decline, sharp turns, short stops).

Maybe you found the sweet spot at a certain point in that long, slow bend?

Could be related to terrain a certain distance/time before that spot in the road. The car may only trigger the warning after the fluid level has stabilized which a flat road would contribute to.

> Long slow bend

That’s probably it.

If I were designing a wiper fluid warning, I would use some sort of fluid level sensor and I would denounce it aggressively: the indicator would only light up if the sensor detected a low level for more than a couple seconds. That way traffic circles, bumps, etc would not cause many false positives. I might even couple it to some kind of acceleration sensor so a warning would not turn on during or shortly after any heavy vibration or acceleration.

A long slow bend would cause a prolonged, steady centrifugal force and/or sideways acceleration due to a banked road, which would defeat these mechanisms.